Russia's massive arms trade with Syria was thrust into the spotlight this week after Secretary of State Hillary Clinton accused Russia of sending attack helicopters to its longtime ally, which has recently stepped up its use of such weapons in attacks that have claimed civilian lives. Russia angrily denied the claims, saying its existing arms contracts with Syria "relate exclusively to air defense."

If true, the helicopter transfer illustrates why the United States and other Western countries have blamed Russia for contributing to the violence in Syria. Russia has repeatedly blocked efforts to impose an arms embargo on Syria and reports have surfaced linking Russia to arms shipments arriving in Syrian ports. According to one recent study, Russia has provided the Assad regime with over three quarters of its major weapons over the past five years.

That massive arsenal is why the State Department said on Wednesday that Russia has Syrian blood on its hands.

"On a daily basis, on an hourly basis, we are seeing Russian- and Soviet-made weaponry used against civilians in towns all across Syria," spokeswoman Victoria Nuland said.

Indeed, while Clinton's remarks were widely interpreted to mean that Moscow was selling Syria helicopters, an official, speaking on the condition of anonymity, told ABC News that there is evidence that Russia has sent refurbished helicopters back to Syria. Arms trade experts say that Russia has a longstanding practice of bringing military equipment sold to other countries back to Russia for repair or upgrades.

"Such repairs/overhauls/modernization is common practice, and that is very likely the explanation," Pieter Wezeman, a senior researcher at the Stockholm International Peace Research Institute (SIPRI), which tracks global arm sales, told ABC News by email.

While stressing that he has seen no evidence to support Clinton's specific allegations, Wezeman noted that in 2009 Syria sent several of its Mi-24 helicopters back to Russia for repairs. If such transfers are taking place today, he said, they are likely taking place far from the public eye.

"Regardless, if true, it would still mean significant Russian involvement in Syrian capabilities to attack rebel forces," he added.

Russia's Massive Arms Trade with Syria

Russia has maintained a lucrative, and growing, arms trade with Syria for years. It is one aspect of a close relationship between Moscow and Damascus that ensures Russia a foothold in the Middle East and access to the Syrian port in Tartus, which gives Russia's navy reach into the Mediterranean Sea.

According to a report released by SIPRI in March, Russia sold Syria 78 percent of its "major" weapons over the past five years. Such major weapons include tanks, aircraft, and air defense systems. SIPRI estimated that current and pending arms sales between Russia and Syria are worth between $5 billion and $6 billion.

According to SIPRI, in 2011 Russia continued deliveries of Buk-M2E surface to air missile systems and Bastion-P coastal defense missile systems. Moscow also secured an order for 36 Yak-130 trainer/combat aircraft. An order of 24 MIG-29M2 aircraft is set to be delivered shortly.

Syria owns a fleet of Russian-made Mi-24, Mi-25, and Mi-17 helicopters which, according to a Russian arms trade expert quoted by RIA Novosti, were sold to them more than 20 years ago by the Soviet Union.

"There is no information regarding new contracts for attack helicopters supplies to this country," the editor-in-chief of the Armaments Export Andrei Frolov said.

Russia is also said to be carrying out similar refurbishments of Syria's stock of Russian T-72 tanks. SIPRI says it's possible some of those tanks were still being shipped back to Syria during the past year of conflict. In 2007 Syria began shipping its 1000 tanks back to Russia, which were being upgraded at a rate of 200 a year, meaning the last batch may have been sent back to Syria in the past year as the conflict escalated. Syria has deployed tanks in recent fighting.

Russia Blocks Arms Embargo

Despite international calls to end its military sales to Syria, Russia has vowed to fulfill its defense contracts, citing the absence of an arms embargo. Such an embargo does not exist largely because Russia has blocked measures, including efforts at the United Nations Security Council, that might impose one.

Speaking at a conference on Tuesday before Clinton's remarks, the deputy CEO of Russia's arms export trading company Igor Sevastyanov said "No one can ever accuse Russia of violating the rules of armaments trade set by the international community."

He said Moscow will continue plans to supply Pantsyr mobile gun and missile air defense systems to Syria. Those systems could not be used to attack civilians or the opposition, but count deter a Libya-style international intervention that Russia remains opposed to.

"The contract was signed long ago and we supply armaments that are self-defense rather than attack weapons, and there can be no talk about any violations by Russia or Rosoboronexport," he said.

Small Arms and Shadowy Shipments

Multiple reports have surfaced in recent months that suggest Russia may also be supplying Syria's armed forces with small arms and ammunition, weapons that could be used in the ongoing conflict.

Russia has denied the reports. President Vladimir Putin himself addressed the matter directly during a press conference in Germany last month, telling reporters: "As for arms supplies, Russia is not supplying arms that could be used in civil conflicts."

Arms trade experts say it's almost impossible to verify such reports, since small arms and ammunition are often much harder to track and are sometimes omitted from ship manifests.

"It is possible that significant volumes of supplies of other arms have occurred without us having noticed," SIPRI's Wezeman said.

Earlier this year Cypriot authorities reportedly found ammunition on a Russian ship bound for Syria. The ship was eventually allowed to proceed to Syria, but it remains unclear what happened to the containers in question.

Last month US Ambassador to the UN Susan Rice slammed Russia over reports that another Russian ship, the Professor Katsman, was suspected of carrying arms and had docked in the port of Tartus. According to a Sunday Telegraph report this week, the ship was owned by a company controlled by Vladimir Lisin -- a Russian oligarch who has offered a $1 million reward for any Russian athlete who wins a gold medal at the London Olympics.

Meanwhile, Russia isn't the only side allegedly arming the conflict. The Independent reported this week that Saudi Arabia and Qatar are sending weapons and ammunition to the Syrian opposition.

SIPRI says confirming such reports on the ground will be very difficult, not least because foreign journalists are largely barred from entering the country.